| Property | STATIC | DYNAMIC-ORINGS |
|---|---|---|
| Groove (gland) compression | 15–30% | 10–20% (less squeeze to reduce friction) |
| Surface finish (Ra) | 0.8–1.6 µm acceptable | 0.2–0.4 µm (reciprocating); 0.1–0.2 µm (rotary) |
| Typical compound hardness | 70 Shore A standard | 70–90 Shore A (harder = less extrusion) |
| PTFE back-up ring required? | Above 10 MPa typically | Less critical — depends on design |
| Lubrication | Not always required | Usually required — reduces friction and wear |
| Key failure mode | Compression set, chemical attack | Wear, spiral failure, extrusion |
Static seals (no relative motion between mating surfaces) can tolerate higher compression and coarser surface finishes. Dynamic seals (reciprocating piston/rod, oscillating, rotating) need tighter surface finishes, lower compression to reduce friction and heat, and often higher-hardness compounds to resist extrusion under pressure. O-Ring kits for maintenance should specify compound hardness and whether the application is static or dynamic.
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